I have two young daughters, 11 and 13. They have both ridden with me many times. The older one has been riding for 4 years, she loves it. Get a good sissybar to keep them from sliding off the back, it also makes them feel more comfortable and safe. I got some nice leather jackets for both of them at Wilson Leather outlet store, on sale too, they outgrew them in a year, need some more now too. I always make them wear good shoes, the leather jacket, gloves (light leather ones are fine, from any farm store) and a ful;l face shield. Little skin will get scarred for life by small rocks, bugs etc. Not worth the risk.
I have never been on a ride any longer than about two hours with them though, they get bored by then and want to get off.
Be sure you have the safety talk with them and remind them EVERY time you ride, "Listen to me, do not lean, do not get on or off unless I tell you to do it, do not stick your arms out to the side, you are not an airplane, the passenger is also requireded to wave at every other motorcycle we see (by far their favorite part), Sit still, no squirming, go the the bathroom BEFORE we leave, pound me on both sides with both hands if I go to fast, if you want to go faster, pound me on both sides with both hands rapidly (I get a lot of that)"
The squeals of delight and the sh1t-eating grins they get are this father's joy. My oldest is destined to be a biker babe for sure. As long as it's on a Triumph, which she also agrees with.